1) Field of the Invention
The present invention relates to an optical scanner used in image forming apparatuses such as digital copiers and color laser printers, and to an image forming apparatus utilizing the optical scanner.
2) Description of the Related Art
In recent years, with the development of high-speed color image forming apparatuses, the so-called four-drum tandem digital color copiers and color laser printers have been put to practical use. In such apparatuses, four photosensitive drums are disposed in a direction in which a recording paper is carried. A latent image is formed by exposing simultaneously by using a plurality of scanning optical systems corresponding to each of the photosensitive drums. The latent image formed is visualized in a developing unit that uses developers of different colors such as yellow (Y), magenta (M), cyan (C), and black (K). A color image is obtained by transferring the visualized images on the same recording paper one after the other such that the images are superimposed.
In the four-drum tandem image forming apparatus, as compared to a one-drum image forming apparatus, the colors can be output with the same speed as in monochrome. Therefore, the four-drum tandem image forming apparatus is advantageous for high-speed printing.
A deflector is used jointly with the scanning optical systems. Light is allowed to be incident on the deflector at different angles of incidence and a width in a sub-scanning direction of the deflector is adjusted to be same as a width of deflection of a single light beam. This enables to reduce the size and cost of the deflector.
To block noise that is developed due to rotation of the deflector and to prevent contamination of a reflecting surface of the deflector, the deflector is covered by a housing or a cover and an inlet for incident light beam and an outlet for emergent light beam are sealed up by a noise proof and dustproof transparent parallel plate.
However, if an angle of incidence of a beam that passes through the parallel plate is wide, the image forming is affected to a great extent by a figure tolerance of the parallel plate. Conventionally, technologies have been proposed in Japanese Patent Application Laid-Open Nos. 2001-290095 and H9-274134 to reduce the effect on the image forming.
Moreover, if the parallel plate is disposed near the deflector, light reflected from the parallel plate becomes ghost light and reaches a surface to be scanned, thereby forming a ghost image. Generally, the ghost image that is formed appears as straight black stripes extending in the sub-scanning direction. Conventionally, technologies to remove the ghost image have been proposed in Japanese Patent Application Laid-Open Nos. 2001-290095, H9-274134, and 2001-318335.